Fix endless loop when adding TabPage directly to Form.Controls

src/System.Windows.Forms/System/Windows/Forms/Control.ControlCollection.cs

@@ -72,6 +72,8 @@ public virtual void Add(Control? value)
             // Add the control
             InnerList.Add(value);
 
+            int _savedTabIndex = value._tabIndex;
+
             if (value._tabIndex == -1)
             {
                 // Find the next highest tab index
@@ -101,6 +103,25 @@ public virtual void Add(Control? value)
                     // you could end up with a control half reparented.
                     value.AssignParent(Owner);
                 }
+                catch
+                {
+                    // AssignParent may throw for invalid parents, leaving the control partially added.
+                    // Roll back the failed add to keep the collection and parent state consistent.
+                    if (InnerList.Contains(value))
+                    {
+                        InnerList.Remove(value);
+                    }
+
+                    // AssignParent may have already changed the parent before throwing.
+                    // Restore the previous parent to keep Parent and Controls collection in sync.
+                    if (value._parent == Owner)
+                    {
+                        value._parent = oldParent;
+                    }
+
+                    value._tabIndex = _savedTabIndex;
+                    throw;
+                }
                 finally
                 {
                     if (oldParent != value._parent && (Owner._state & States.Created) != 0)

src/test/unit/System.Windows.Forms/System/Windows/Forms/Form.ControlCollectionTests.cs

@@ -23,4 +23,38 @@ public void ControlCollection_Ctor_NullOwner_ThrowsArgumentNullException()
     {
         Assert.Throws<ArgumentNullException>("owner", () => new Form.ControlCollection(null));
     }
+
+    [WinFormsFact]
+    public void ControlCollection_Add_InvalidTabPageParent_DoesNotLeaveHalfAddedControl()
+    {
+        using Form form = new();
+        using TabPage tabPage = new();
+
+        int oldCount = form.Controls.Count;
+
+        Assert.ThrowsAny<Exception>(() => form.Controls.Add(tabPage));
+
+        Assert.Equal(oldCount, form.Controls.Count);
+        Assert.False(form.Controls.Contains(tabPage));
+        Assert.Null(tabPage.Parent);
+    }
+
+    [WinFormsFact]
+    public void ControlCollection_Clear_AfterFailedTabPageAdd_DoesNotHang()
+    {
+        using Form form = new();
+        using Button button = new() { Name = "button1" };
+        using TabPage tabPage = new();
+
+        form.Controls.Add(button);
+
+        Assert.ThrowsAny<Exception>(() => form.Controls.Add(tabPage));
+
+        // If the failed add left the TabPage half-added,
+        // Clear() could hang. After the fix, it should complete normally.
+        form.Controls.Clear();
+
+        Assert.Empty(form.Controls);
+        Assert.Null(tabPage.Parent);
+    }
 }